Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “Small Cell” as used herein is defined as a low-powered radio access node, or base station that operates in licensed spectrum that have a range of 10 meters to 1 or 2 kilometers, compared to a mobile macrocell which might have a range of a few tens of kilometers. Small cells are a vital element to 3G/4G data off-loading, and many mobile network operators see small cells as vital to managing Long Term Evolution (LTE) advanced spectrum more efficiently compared to using just macrocells. The primary use of small cells is to increase capacity of the traffic for the operators rather than increase the mere coverage of the network.
The term “multiple-input-multiple-output” or “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver. MIMO systems may improve communication performance by offering significant increases in data throughput and link range without additional bandwidth or increased transmit power. MIMO systems may achieve this goal by spreading the transmit power over multiple antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per frequency range or hertz (Hz) of bandwidth) or to achieve a diversity gain that improves the link reliability (e.g. reduced fading), or increased antenna directivity.
The term “multi-user multiple-input-multiple-output” or “MU-MIMO” as used herein is defined as a wireless communication system in which available antennas are spread over a multitude of independent access points and independent radio terminals—each having one or multiple antennas. In contrast, single-user MIMO considers a single multi-antenna transmitter communicating with a single multi-antenna receiver. To enhance the communication capabilities of all terminals, MU-MIMO applies an extended version of space-division multiple access (SDMA) to allow multiple transmitters to send separate signals and multiple receivers to receive separate signals simultaneously in the same frequency band.
The term “Time-division duplexing” or “TDD” as used herein is defined as is the application of time-division multiplexing to separate outward and return signals. It emulates full duplex communication over a half-duplex communication link. Time-division duplexing has a strong advantage in the case where there is asymmetry of the uplink and downlink data rates. As the amount of uplink data increases, more communication capacity can be dynamically allocated, and as the traffic load becomes lighter, capacity can be taken away. The same applies in the downlink direction. For radio systems that aren't moving quickly, hereinafter referred to as “quasi-static” stations, another advantage is that the uplink and downlink radio paths are likely to be very similar. This means that techniques such as beamforming work well with TDD systems.
In some MU-MIMO that are already known in the art, a spatial separation mechanism creates multiple separated channels between the base station and the users or the same spectrum; sub sets of users population are assigned to these different spatial channels; a common basestation scheduler makes sure that simultaneous users that may experience MU-MIMO's self-inflicted cross talk, are served over non overlapping PRBs, thus maintaining efficient spectrum multiplexing.
According to prior art MU-MIMO systems, the aggregated data rate of an N-user MIMO is slightly below N times the data rate of a Single-User-MIMO (SU-MIMO), due to random distribution of the users, channel estimation errors, mobility, and the projected signal levels dependence on pairing of users, which impacts MCS estimation accuracy. Such MU-MIMO base stations serve as N unified legacy base stations, sharing common channel estimation and common MIMO processing blocks, as well as common Radio Resources Control (RRC) and a common scheduler.
FIG. 1A is a block diagram of a system 100 and illustrating the function stacks of single-user MIMO system according to the prior art. FIG. 1B is a block diagram illustrating the function stacks of a multi-user MIMO system, according to the prior art. For SU-MIMO, system 100 may include: higher media access control (MAC) 110 may perform scheduling for the MIMO operation; lower MAC 120 which may handle (for example, multiplex, de-multiplex, modulation, and demodulation) the multiple data streams for the MIMO; pre-coding function 130 which may transmit each of the multiple data streams through the multiple transmit antennas according the pre-coding weight.
In order to upgrade the SU-MIMO system into a MU-MIMO system, one may modify the scheduler in higher MAC 110 to a new higher MAC 160 of system 150 to coordinate the multiple users. Additionally, one may multiply the data handling function in lower MAC 120 to newer lower MAC 170, of system 150, to accommodate more users simultaneously. Furthermore, one may modify the pre-coding function 130 to the newer one 180 as in system 150 so that each data stream from all MU-MIMO users may transmit through all the transmit antennas simultaneously.